Fluid venting structures are used to evaluate fluid migration in the subsurface and vertical changes in their morphology reflect variations in the intensity of fluid leakage through time. This research uses high-resolution 3D seismic data from the Lower Congo Basin offshore Angola to analyse complex assemblages of vertically-stacked fluid venting systems in the Middle Miocene to Holocene succession. Individual fluid-venting structures that form part of vertical venting systems include conical pockmarks, fluid related shallow depressions with flat bottom, chimney structures and positive high amplitude anomalies (PHAAs). Detailed seismic interpretation reveals for the first time that chimneys and PHAAs have a variety of plan form geometries (circular through to linear) within a given vertical succession of fluid venting structures. Linear chimneys are often associated with PHAAs which are interpreted as deposits of methane-related carbonate. The geometry and depth of depressions associated with fluid venting structures are used to infer relative rates of fluid flux or intensity of the fluid eruption. This classification scheme is as follows; linear PHAAs and conduits (slow fluid venting), sub-circular PHAAs and shallow depressions (slow to moderate rates of venting), pockmarks (fast rates of fluid venting). Two new types of pockmarks are identified based on the architecture of the sediments which infill them. They include advancing pockmark arrays and nested pockmarks. In contrast to normal pockmarks which are stacked vertically, successions of nested pockmarks and advancing pockmark arrays are laterally offset and migrate laterally, typically downslope. The reactivated craters of advancing pockmarks erode the downstream margin of preceding and underlying infill sequences whilst the infill sequence of nested pockmarks migrate gently downslope but without eroding the underlying and preceding infill sequence. Nested pockmarks and advancing pockmark arrays are confined to inclined surfaces. Downslope migration is a product of the interplay between slope inclination, sedimentation rate and bottom current activity. The trails of nested pockmarks and advancing pockmarks cluster above the axis of gas-bearing turbidite channels. PHAAs, chimneys, a present day bottom simulating reflector and negative high amplitude pockmark infills also occur in these areas and pockmarks occur above crestal faults which root in underlying rollover anticline. This implies the fluid source was derived from depth in the turtle anticline structure. The presence of negative, high-amplitude pockmark infills may suggest the fluid source was gas. A detailed spatial analysis and characterization of fluid venting structures on successive horizons in the middle Miocene to Holocene succession indicates that their distribution and type are affected by tectonic structures and vertical changes in the nature of the host sediments. Linear chimneys occur vertically below Linear PHAAs. The former are occur within the polygonally faulted interval whilst the later occur at the top of above the polygonally faulted interval. They tend to cluster in parts of the basin where the orientations of polygonal faults are strongly perturbed such as around salt diapirs and in salt-withdrawal synclines. Both linear venting structures are interpreted to post-date polygonal fault growth. Linear chimneys and linear PHAAs both have a close spatial and geometric relationship with PFs and deeper extending salt-related faults. The parallel relationship between linear venting structures and adjacent faults (salt or compaction related) are attributed to development and alignment of vertical hydraulic fractures (vertical conduits for linear chimneys) in the local fault induced stress field which subsequently provides fluid migration pathways. A model of vertical fluid migration through the polygonally faulted interval is proposed it involved initial fault-bound trapping, sealing and overpressure beneath an impermeable horizon in the lower part of the PF tier in the early stages, and vertical breaching, hydraulic fracturing and vertical fluid rise through the upper part of the tier in the later stages. Vertical changes in the morphology and type of fluid venting structures occur across small vertical transitions which reflect changes in gross lithology from fine-grained hemipelagites to chaotic and heterogeneous mass transport deposits (MTDs). A linear zone of positive high amplitude anomalies, referred to as a linear venting network, transitions to an array of elongate-to-sub circular shallow depressions with flat bases or conical pockmarks at the upper surface of MTDs. Further changes occur above MTDs where honeycomb pockmarks, so-called based on their hexagonal-shaped perimeter which coincides with polygonal fault intersections, exist. In these cases vertical changes in the type of fluid venting structures are attributed to contrasting patterns of mechanical failure in different sediment when subject to fluid overpressure. Although individual linear chimneys and PHAAs post-dating polygonal growth are strongly affected by the location and orientation of PFs those which precede polygonal fault growth such as pockmarks can affect the orientation of PFs. For example deep pockmark craters with the steepest sidewall inclinations coincide with overlying concentrically aligned PFs yet those which are shallower and have gently dipping sidewalls coincide with more isotropic PF patterns. This suggests that the topographic relief of pockmark craters or compaction above craters of certain depths perturb the stress state within sediments where polygonal faults form. This thesis has demonstrated that seismic interpretation of vertical successions of different types of fluid venting structures can be used to reconstruct spatial variations in the intensity of fluid flow at different stages in the evolution of basins.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:610987 |
| Date | January 2013 |
| Creators | Ho, Sutieng |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/59722/ |
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